Color television transmission systems used throughout the world are based on or derived from signal specifications originally defined in the United States by the National Television Systems Committee (NTSC). Such systems, which will be referred to herein as NTSC-type systems, include the NTSC format system used in the United States and the well-known PAL and SECAM systems used abroad. These systems utilize composite color television signals comprising a wide band monochrome signal and a plurality of chrominance signals (sometimes referred to as color difference signals).
The wideband monochrome signal, generally represented by the term Y', is typically a combination of three primary color signals, e.g., red, blue, and green, which have been precorrected for the power law gamma characteristic of typical display tubes. The presence of pre-correction in the constituents of a signal is conventionally indicated by designating the signal with a prime ('). The monochrome signal is typically of the form Y' = .SIGMA.A.sub.c C'=A.sub.r R'+A.sub.g G'+A.sub.b B', wherein C' represents any gamma-corrected primary color signal, A.sub.c, A.sub.r, A.sub.g and A.sub.b represent respective nominal relative luminance coeffiencts for primaries, and R', G', and B' represent the gamma-corrected color signals for primaries of red, green, and blue, respectively. The monochrome signal Y', as defined herein, should not be confused with the colorimetric luminance Y which is a corresponding combination of the uncorrected primary signals, nor should it be considered equal to a gamma-corrected luminance signal because in the monochrome signal, it is the individual primaries which have been corrected, not the entire combination, i.e., Y'=.SIGMA.A.sub.c C' is not uniquely related to Y=.SIGMA.A.sub.c C for typical gamma correction exponents.
The chrominance signals in NTSC-type systems typically comprise signals representing the difference between a gamma-corrected primary color signal and the monochrome signal or a linear combination of such color difference signals. Specifically, color difference signals can be generally represented by the term (C'-Y').sub.L wherein C' represents any gamma-corrected primary. The subscript L serves as a reminder that the chrominance signals are typically transmitted with a bandwidth which is relatively reduced as compared with the Y' signal and that it may be further bandwidth reduced at the receiver
Typical NTSC-type transmission systems are designed to transmit Y' in its full wide bandwidth and linear combinations of the chrominance signals in reduced bandwidth. In the United States, for example (R'-Y'), (B'-Y'), and hence (G'-Y') are transmitted in linear combination chrominance signals designated the I' chrominance signal and the Q' chrominance signal. The linear combination chrominance signal is measures on a particular phase of the chrominance subcarrier known as a chrominance axis. Hence, for example, the I' and Q' signals define separate chrominance axes. While the I' and Q' signals have somewhat different bandwidths, each substantially narrower than that of the Y' signals, the excess portion of the relatively wider bandwidth I' signal is often lost at the receivers, most of which are designed for equiband operation.
Conventional receivers use either equiband chrominance for all axes, or use in varying degrees the added intermediate bandwidth I' signal transmitted as a single-sideband component.
Some receivers use simplified approximations to the nominal I' passbands, while other receivers use wideband equiband systems. In order to shorten the chrominance transient epoch, these receivers accept erroneous chrominance components nominally from the single-sideband I' components, and they variously proportion these erroneous components between the I' and Q' channels.
This specification will present the equations and circuit means relative to the substantial chrominance improvements of this invention, first for processing of equal band signals and then also for processing of I' and Q' chrominance signals of unequal bandwidths.
Common NTSC-type receivers demodulate and matrix the received chrominance signals into a plurality of reduced bandwidth chrominance signals (C'-Y').sub.L. The receiver then effectively adds the monochrome signal Y' to each chrominance signal in order to derive a plurality of signals which include, respectively the low frequency components C.sub.L ' of the primary color signals generated at the color camera and a combined high frequency component. The low frequency primary color components are sometimes referred to as the large area color signals. The high frequency monochrome components, Y.sub.H ' is generally referred to as the mixed highs signal because it is transmitted and displayed only as a specific combination of the high frequency primary color components.
It has long been recognized that conventional NTSC-type receiving systems exhibit a number of visible color infidelities upon display, particularly in regions of sharp transitions from one color to another or, within a single color, in transitions from one luminance to another. When conventional NTSC-type receiver displays are compared against a reference display in which all of the primary color signals have a wide bandwidth comparable to that of Y', visible color infidelities, such as incorrect highs, polarity reversals, and errors in visual luminance can be observed in regions of sharp color transitions. These infidelities are clearly visible on modern displays as resolution and luminance errors, chromaticity smear, local desaturation, and luminance notches.
In step transients within a single primary color, the highs can be too small relative to the lows.
In step transients involving more than one primary, the highs can be of incorrect amplitude to accurately reproduce even a single colorimetric coordinate such as the luminance, Y. Furthermore, because of the change in luminance, .DELTA.Y is .SIGMA.A.sub.c .DELTA.C whereas .DELTA.Y' is .SIGMA.A.sub.c .DELTA.C', the high frequency components of Y' are sometimes of such polarity that the reproduced visual luminance Y on a step has upside-down highs. This infidelity occurs on any step wherein .DELTA.Y' is of one polarity and .DELTA.Y is of the other.
In step transients from a first primary in one region to a second primary in a horizontally contiguous region, with the third primary small or absent, a conventional color television display will exhibit in the high frequency portion of the step: (1) amplitude errors in the high frequency portion of each primary; (2) reversed polarity in the high frequency component of one primary; and (3) possible rectified high frequency components in another primary, producing desaturation and spurious low frequency components therein.
And in transitions between any substantially saturated color and another color which is substantially its colorimetric complement, there appear spurious observable dark regions commonly known as luminance notches.
In regions of significant color saturation, a conventional color television display will typically exhibit: (1) a loss in detail due to inadequate high frequency components in the one or more strong primary colors and (2) over-modulation and rectification due to an excess of high frequency signal components in the one or more weak primary colors. The simplest example is that of a single saturated primary. In such case, the transmitted high frequency signal available for that primary solely from the high frequency signal Y' is too small, while the same Y' signal components are excessive in the other primary colors. More generally, similar infidelities generally occur and tend to be visible whenever the local color deviates significantly from white. Such infidelities are clearly visible on modern color television displays as resolution and luminance errors, chromaticity errors, and sometimes as spurious low frequency errors and local desaturation.
In common television pictures conventionally reproduced colorimetric errors of the types cited may concurrently occur differently in different parts of the frequency spectrum and also at different points in the picture.
In conventional receivers which receive at least monochrome Y' portions of the transmitted signal, when color steps occur which include transmitted single sideband I' components, the transmitted single sideband I' components extend down significantly into the spectral region from which the Y' signal is extracted for picture reproduction and may therefore be rendered visible by rectification and by the presence of noticeable patterns. This color infidelity is particularly visible on extended, near vertical edges. Use of multiline signal combination techniques tends to shift the interference to other patterns such as edges of different slope, or to produce patterns at subharmonics of field rates.
While there has been a widespread recognition that the conventional reception and display of NTSC-type signals produce the above-described color infidelities, none of the receiver correct circuits proposed in the prior art have provided satisfactory results. Typical prior art proposals for reducing such distortions have allocated the largest portion of the fault to the use of a Y' signal on transmission instead of a true luminance measure, such as a gamma-corrected Y signal. Accordingly, these proposals have included the proposal to change the transmitted signal from Y' to Y to the inverse-gamma power and various other proposals to otherwise precorrect the transmitted monochrome signal. All such proposals have gone unaccepted in the industry because (1) they typically failed to provide adequate color correction; (2) they typically degraded image quality in other respects; and (3) they were, in many cases, unduly complex.
The specific problem of inadequate highs has been treated, but the proposed solutions have deteriorated the image quality in other respects. For example, some prior art receivers utilized enhanced gain in the common mixed-high region of the monochrome signal. This approach, however, cannot provide polarity corrections; cannot provide the differential relative amplitudes needed in the individual primary colors; and degrades the display image by producing increased rectification and desaturation. It has also been proposed to modulate the common mixed highs by the ratio of the square of an estimated gamma-corrected luminance to the square of Y'. This proposal, also, fails to provide polarity correction and differential relative amplitudes, and it would introduce a major increase in rectification and desaturation as well as generate spurious high frequency signals.